GB2205828A

GB2205828A - Methods of manufacturing polarisation-maintaining optical fibres

Info

Publication number: GB2205828A
Application number: GB08800275A
Authority: GB
Inventors: Hiroshi Kajioka; Koudo Yamada; Yuuetsu Takuma; Tatsuo Teraoka
Original assignee: Hitachi Cable Ltd
Current assignee: Hitachi Cable Ltd
Priority date: 1987-06-15
Filing date: 1988-01-07
Publication date: 1988-12-21
Anticipated expiration: 2008-01-07
Also published as: GB8800275D0; US4859223A; GB2205828B

Description

2 2 0 2 METHODS OF MANUFACTURING POLARISATION-MAINTAINING OPTICAL FIBRES:

This invention relates to methods of manufacturing polarisation-maintaining optical fibres, and particularly polarisation-maintaining optical fibres suited for use in coherent communication systems.

In general, there are two kinds of polarisation-maintaining optical fibres, namely: a fibre having a circular core to which a stress is applied, such as an elliptical-jacket fibre, a PANDA fibre, or a Bow-tie fibre; and another kind having a non-circular core, such as an elliptical or rectangular core. In such polarisation maintaining optical fibres, the phase constant difference AC between naturally cross-polarised modes is widened in order to reduce any cross-talk of these modes to a minimum, and so obtain a polarisation-maintaining characteristic.

In Figures 1. and 2, two known prior art methods of manufacturing the kind of polarisation-maintaining optical fibres whose cores are not circular in cross-section are shown: Figure 1 relating to the Japanese Patent Laid Open No 24306/81; and Figure 2 relating to Japanese Patent Laid Open No 92505/81.

According to the known method illustrated in Figure 1, the first step is the fabrication of a core member comprising a base material 113 which includes a circular core 111 and circular cladding 112, and of an outer member comprising a silica glass tube 121 whose central portion is a circular cavity and whose outer cross-section is oval (Figure la). Next, the base material 113 is 2 inserted into the silica glass tube 121 to form a combined body, and this combined body is heated externally to form a preform 130 (Figure lb). This preform 130 is then heated and wire-drawn, so as to create an optical fibre including a fibre jacket 143 whose outermost profile of the section is approximately circular due to surface tension, and contains the core 141 and the cladding 142), both of which now have elliptical cross-sections (Figure 1c).

In such a manufacturing method, with the use of a silica glass tube 121 in this way, unAesirable limitations are imposed upon the diameter, which is prone to be limited to a certain degree.

Accordingly, a preform of large diameter cannot be readily obtained, nor can a long optical fibre, so that it is not suitable for mass production. Furthermore, since the core 141 is transformed from a circular cross-section to an oval, through the process of heating and wire-drawing, it is very possible that the configurations of the core 141 and the cladding 142 will vary, especially with fluctuations of the heating temperature and the viscosity of the glass, for example. Hence it is difficult to fabricate an oval core optical fibre of predetermined design configuration with high efffciency, and the production of the fibres with precisely the same configuration is also difficult.

In the alternative prior art method shown in Figure 2, (Japanese Patent Laid Open No 92505/81), firstly a vitreous tube 201 of circular cross-section and a core member 203 having a rectangular cross-section are prepared, the core member 203 being shaped by mechanically grinding a round glass bar (Figure 2a). Then a vitreous 3 cladding layer 202 is formed on the inner surface of the vitreous tube 201 by means of M (Chemical Vapour Deposition) (Figure 2b).

The core member 203 is then inserted and fixed in the glass tube 201 with its internal vitreous cladding layer 202, the core member 203, and the glass tube 201 being aligned co-axially (Figure 2c). The glass tube 201 and the core member 203 are then rotated together and heated from outside to shrink them both, so that the core member 203 and the vitreous cladding layer 202 are blended and combined, creating a preform 205 (Figure 2d). This preform 205 is then wire-drawn to give an optical fibre which has a similar cross-section to the preform 205.

In this method, as for the previous method shown in Figure 1, the use of a glass tube 201 imposes some limitation on the diameter of the preform, and high-volume production capability is very difficult to achieve. Furthermore, at the process of fabricating the preform by shrinking the glass tube 201, the core member 203 and the glass tube 201 may both easily change their form, so"that it is difficult to manufacture the desired optical fibre with both a high yield and high precision.

Another prior art method of manufacturing a polarisation- maintaining optical fibre of the elliptical core type is disclosed in the publication of Japanese Patent Laid Open No. 130044/79.

With regard to performance, even if the cross-talk of the polarisation-maintaining optical fibre is small, that is to say it has an excellent extinction ratio of, for example, -30 dB/km, in an 4 actual case the extinction ratio is reduced to some -10 dB/km after 100 km transmission, so that the modes whose cross-talk become high may function as an echo to the main signal and be transmitted to the receiver. Therefore, the transmission quality will be reduced.

In order to prevent such a deterioration in the transmission quality, there is a theory that the aforesaid stress-applied fibre and a non-circular core fibre could be so combined together as to compensate for any propagation velocity difference between orthogonally crossed polarlsed modes because of multi-refractive indices generated by the applied stress. However, no suitable method of manufacturing thereof has been found to date.

One object of the present invention is to provide a method of accurately manufacturing a polarisation-maintaining optical fibre having a non-circular core using mass-production techniques and to yield a polarisation-maintaining optical fibre capable of preserving the transmission quality over long distance transmissions.

The method proposed in accordance with the present invention comprises the steps of:

transforming a section of a circular rod core to give it a non-circular cross-section by removing portions of the rod at two opposed surfaces by an initial shaping process; forming a preform by sintering glass particulates accumulated on the surface of the resultant non-circular rod core; and heating and wire-drawing said preform in a further process step.

The invention will now be described with reference to the drawings, in which:- Figures 1 and 2 are process drawings showing prior art methods of manufacturing polarisation-maintaining optical fibres of the kind having a non-circular core; Figure 3 is a process drawing showing the method of manufacturing a polarisation-maintaining optical fibre in accordance with the present invention; Figure 4 is a set of explanatory graphs related to a section of a preform of Figure 3, to indicate the reefractive index contributions in the directions of both the X and Y axes; Figure 5 is a graph showing the relationship between wavelength and transmission loss of an optical fibre that has been formed by heating and drawing the fibre preform to which Figure 4 relates; and Figure 6 is a a process drawing showing one alternative method of manufacturing a polarisation-maintaining optical fibre in accordance with the invention.

The prior art methods shown in Figures 1 and 2 have been discussed at length herein above. Preferred embodiments of this present invention will now be described with reference to Figures 3 to 6.

6 Figure 3 shows a a process drawing of a polarisation- maintaining optical fibre forming a first exemplary embodiment constructed in accordance with this invention.

In a first step, GeC14 and SIC14 are supplied from a core burner 1 at amounts of 20 mg/min and 800 mg/min, respectively, while only SIC14 of 5 g/min is supplied from a cladding burner 2.

Glass particulates are accumulated by VAD, (Vapour Phase Axial Deposition) forming a porous base material 5 constituted by a core 3 that is 30 mm in diameter and a cladding 4 that is 120 mm in diameter, see Figure 3a. The particulates of glass may be accumulated by 0M (Outside Chemical Vapour Deposition).

In the next step, this porous base material 4 is sintered, being transformed to a vitreous state in an environment including fluorine, namely, the environment of S! F4 at 170 ml/min and He at 10 1/min, to produce a core rod 8 that is 60 mm in diameter with a cladding 7, as shown in Figure 3b. At this time, fluorine is supplied in a manner such that the specific refractive index of the cladding 7 is decreased by approximate-ly 0.22% in comparison with silica. Instead of grinding a vitreous rod, the shaping can be performed on a sootlike rod by using a cutter heated to a high temperature. After that, the vitreous core rod 8 is lengthened in the next step to leave a diameter of 25 mm, two opposed sides 9 of the cladding 7 are mechanically ground away some 970 mm in the axial direction thereof, and the thus formed surface is abraded by a grinder to leave a 25 mm x 12 mm cross-section, and the faces are fire-polished by an oxyhydrogen burner at a temperature of 15000C, giving the cross-section shown in Figure 3c.

7 Then, around the core rod 8, S102 glass particulates are vapour-deposited from outside for a support (Figure 3d), and it is sintered at 15000C to be vitreous. This process is repeated again being finished at a diameter of 30 mm, i.e. at first the outside vapour deposition is conducted until the diameter of the rod reaches 1.5 times the original and the drawing is conducted until the diameter is reduced to 30 mm, and then another outside vapour deposition step is performed until 1.8 times the thus formed rod, and then lengthened to leave a 30 mm diameter.

In this manner, a preform 12 which comprises the core 6, the cladding 7, and the support 11 is created, as shown in Figure 4a, with the glass particulates transformed to transparent glass. Here, at sintering of glass particulates 10, a shrinkage force works on the core 6 and the cladding 7, shaping them into ovals, in a manner such that they have orthogonal major axes.

The refractive index distributions of this preform 12 in the X and Y axis directions were examined by means of a preform analyser, and the results are shown in Figures 4b and 4c. They depict that the specific refractive index difference An+ of the core 6 compared with silica (the support 11) is 0.23%, while that An- of the cladding 7 compared with the silica is -0.22%. This means that fluorine was also added to the-core 6.

In the X axis, the diameter Dx is 29.5 mm; the ratio of the cladding width tx to the core radius ax is 1.0; and the ratio of the support width Tx to the core radius ax is 6.4; while, in the Y axis direction, the diameter Dy is. 30.7 mm, the ratio of the cladding width tY to the core radius aY is 3.4; and that of the support width TY to the core radius aY is 9.0.

8 The final polarisation-maintaining optical fibre is formed by heating the preform at 21000C and wire-drawing at 20 gf to leave a diameter of 150 pm. Figure 5 shows the examination result for 1000 m fibre of the above-mentioned type in lossy wavelength performance.

In the Figure, the cut-off wavelength is 1.35 pm, the transmission loss is 0.21 dB/km at a wavelength of 1.55 pm, and the coupling length is 17 mm. The ellipticity of both the core and of the cladding is approximately.?5%.

In this particular embodiment, the core is not directly shaped into an oval, and Ge02 of high density is not applied to the core when the core is being formed to an oval, so that low loss is obtained in transmission.

Furthermore, since the dispersion between two perpendicu- larly crossed polarised modes (polarisation dispersion) is also small, this optical fibre can be utilised as a transmission path for coherent communication.

In an actual manufacturing process according to the.

above-described embodiment, by supplying GeC14 at a rate ' of 100 m91min and SIC14 at 800 mg/min, which are doped to the core 6 of the porous base material 5 by as rnuch as 1% approximately, and increasing the depth of grinding of the core rod 8, so as to shape the section to 25 mm x 9 mm, a polarisation-maintaining optical fibre having a coupling length in the order to several mm was realised.

Another exemplary embodiment of this invention will now be explained with reference to Figure 6.

9 In Figure 6a, a rod 21 whose diameter is 20 mm will be a central element. It is made of silica glass doped with Ge of approximately 10 mol %, and formed by VAD.

Two opposed portions of the rod 21 are ground to 20 mm x 8 mm in section in the longitudinal direction thereof, as shown in Figure 6b, forming the core 22 with its ground surfaces 23.

To the core 22, which has a rectangular section, cladding soot 26 of pure silica is vapourdeposited from outside by CW or VAD, so that the diameter thereof becomes 120 mm, and then it is transformed to a vitreous state, forming a cladding 24 having a diameter of 60 mm (Figure 6d). The processes of vapour deposition of cladding soot and transforming it to a vitreous state can be repeated more than once.

After that, the cladding is shaped to a base material by grinding the cladding at two opposed portions of the surface, being shaped to a configuration analogous to the core 22 shown in Figure 6e, with its ground surfaces 25.

The thus formed base material is then drawn at a tensile force of 50-60 9f, while maintaining the configuration analogous to that before drawing. By coating with silica, ultraviolet hardening resin, or the like, onto the wire-drawn base material, a primary coated fibre is obtained. The wire-drawing is performed in a manner such that the core meets the requirements of a single mode in terms of diameter.

In this embodiment, the cladding 24 may contain a dopant such as fluorine.

According to the above-mentioned embodiment, it is possible to manufacture at high yield a polarisation-maintaining optical fibre including a core and cladding which are formed to be non-circular in cross-section, for use as a transmission path between optical integrated circuits, for example.

As chemical vapour deposition such as VAD and machining are both employed, the base material can be made in a large scale process, resulting in cost reductions of significant size.

Moreover, since machining is utilised in the manufacturing, the ellipticity of the core can be set to a high value, so that the double refractive index can also be set high, improving the polarisation-maintaining characteristics.

In a sintering process where there is provided a cladding around the core, and the rod core is made non-circular by removing some portions of the cladding by machining, when the glass particulates accumulated on the outer surface of the core rod are sintered, a shrinking force works on the rod. On the other hand, the rod core has been partially cut at its two opposed portions of the surface in the longitudinal direction, that is to say, the rod core has already had an asymmetrical section. Thus, the shrinking force applied to the rod also has an asymmetrical contribution.

Accordingly, the core and the cladding will both be oval in shape, with their major axes orthogonal.

By forming the cladding into an elliptical one, the longitudinal direction of the core becomes the fast axis with regard to refractive index. 'On the other hand, since the core diameter is large, the length of the optical path becomes long. Thus, 1 k 11 polarisation dispersion between two orthogonal polarised modes, with proper parameters, will be greatly reduced, compared with an optical fibre of conventional type. Therefore, the pulse width expansion will be reduced, enabling a wide-band transmission. In other words, even if an extinction ratio is reduced during long distance transmission, accompanying some transmission loss, the generation of any pulse echo is suppressed, so that the general deterioration of transmission quality is significantly reduced.

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Claims

CLAIMS:

1. A method of manufacturing a polarisation-maintaining optical fibre comprising the steps of:

transforming a section of a circular rod core to give it a non-circular cross-section_by removing portions of the rod at two opposed surfaces by an initial shaping process; forming a preform by sintering glass particulates accumulated on the surface of the resultant non-circular rod core; and heating and wire-drawing said preform in a further process step.

2. A method of manufacturing a polarisation-maintaining optical fibre as claimed in Claim 1, wherein said circular rod core is fabricated by vapour phase axial deposition.

3. A method of manufacturing a polarisation-maintaining optical fibre as claimed in Claim 1 or Claim 2, wherein said circular core is fabricated to have a vitreous structure by sintering a porous base material in an atmosphere including fluorine, the porous base material being made by a vapour-phase axial deposition process and including a core portion and a surrounding cladding.

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4. A method of manufacturing a polarisation-maintaining optical fibre as claimed in any preceding Claim, wherein said circular rod core comprises a cladding around a central core portion, and in said process of transforming said section to have a non-circular cross-section, a portion of the cladding is removed by a machining operation.

5. A method of manufacturing a polarisation-maintaining optical fibre as claimed in any preceding Claim, wherein said machining operation for transforming said section of rod core to have a non-circular cross-section includes grinding and fire-polishing.

6. A method of manufacturing a polarisation-maintaining optical fibre substantially as described with reference to Figure 3 or Figure 6.

Published 1988 at The Patent Office.Statc House. 6671 High 'Holborn. London WCI.R4TP. Further c,-)ies maybe obtainedfrom The Patent Office, Sales Branch. 81. I-Aa-y Cray, Orpington. Kent BR5 3RD. Printed by Multiplex techniques ltd. St Mary Cray, Kent. Con. 1.'87